Emulsions
Emulsions are dispersed systems consisting of two immiscible liquid phases (oil and water), where the dispersed or internal phase is finely divided and distributed in another external or continuous phase.
The conventional emulsions are classified in oil/water (O/W) when the oil droplets are dispersed in water and water/oil (W/O), when water droplets are dispersed in oil. It is important to clarify that by “oil” is considered all nonpolar hydrophobic liquids and by “water” is considered all highly polar hydrophilic liquids.
In principle, oil and water phases are made up of only one substance, however, in cosmetic preparations, each phase usually contains a variety of “components”. The viscosity of the emulsion and its appearance are controlled, in part, by the type of emulsion, the particle size of the dispersed phase and the ratio between the internal and external phases. The system is thermodynamically unstable; the particle size can vary once the internal phase constantly tries to agglomerate and separate forming a second phase.
Polymers and emulsifiers are used to delay this inevitable separation. During the process of the emulsion formation, these compounds can be adsorbed in the oil-water or water-oil interface. Depending upon the agent used and by the reduction of interfacial tension, it promotes the stabilization of finely divided droplets and controls the type of emulsion formed (W/O or O/A).
Organogels
Organogels are defined as thermoreversible three-dimensional gel network that immobilizes an organic fluid creating a material with characteristics and rheological properties of a solid, but with greater part of its composition being liquid. They can also be defined as a viscoelastic material composed by a structuring agent and a nonpolar liquid phase. They are semisolid systems where an oil phase is immobilized by a self-assembled three dimensional network formed by the structuring agent. The liquid phase may be a polar organic solvent (such as benzene, hexane, etc.), mineral oil or vegetable oil.
Several chemical compounds are able to structure organic liquids forming organogels. Some examples are sterols, lecithins, mono- and diglycerides, lecithin mixtures with sorbitan esters, fatty acids and fatty alcohols and waxes and wax esters.
The potential applications of organogels in food, pharmaceutical, cosmetic and petrochemicals industries have raised the interest in this type of material. The interest is partly due to the great diversity of possible mesoscopic and microscopic structures. In the food industry there is a great potential for the use of organogels. It can be used to minimize oil migration in food with multiple components, for example, chocolates. Can also be used to structure edible oils, consequently reducing the use of saturated and trans fat. The organogel technology has also been applied in the pharmaceutical field to structure dermal and transdermal products and as transporter or carrier system for topical drugs or therapeutic substances. In the petrochemical industry there are investigations to study the possibility to contain oil spills using gelation phase of the selective organogels, and also gelation of flammable solvents for storage and transportation. They have been also used in the preservation of works of art, where the solvents used to clean oil paintings are gelled in order to prevent its penetration in original layers of paint and prevent damage to the work.
The structuring of oil has recently been applied in the cosmetic industry as structurers for personal care products. It is also known that waxes are applied in structuring oily bases for lipstick, “gloss”, eyeliners, lip balms, etc. Structured emulsions are part of a different scope of organogels. These emulsions refer to structured oils by structuring agents, which, due to their amphiphilic nature, allow the incorporation of a significant amount of water.